Well-defined, periodic nanostructures have received considerable attention since they can be served as useful templates and scaffolds for nanodots, nanowires, magnetic storage media, semiconductors, optical devices, polarizers, and photonic materials [1]. For this purpose, bottom-up approaches have extensively been studied because they can offer an efficient, cost-effective strategy to overcome the technological and economic limits associated with large-scale top-down approaches [1k]. The self-assembly of block copolymers (BCPs) [1k,2], one of the most promising candidates for this purpose, have widely been studied as the sizes, spacings, and morphologies of the nanostructures from the self-assembled BCPs can be simply tuned by varying molecular weight and composition ratio of BCPs and, more importantly, the versatilities in the properties of the blocks can be easily introduced by many well-known chemical techniques.
For some practical applications like polarizers and photonic band gap materials for visible wavelengths, the alternating domain spacing of the self-assembled BCPs usually has to be up to a few hundred nanometers. Thomas and coworkers utilized the partially cross-linked, conventional BCPs to prepare the photonic band gap materials for visible wavelengths [3], but this normally requires the molecular weight (MW) of BCPs to be extremely large for the applications mentioned above. It is noted that, according to the model system for polymers with the MW over the critical entanglement MW [4], the viscosity of polymers gets higher abruptly as the MW gets larger due to polymer chain-entanglement, which yields a significant kinetic barrier for the effective self-assembly of conventional BCPs with high MW [2c, 3, 5]. For this reason, the defects might not be able to be effectively annihilated even upon longer annealing time due to the entanglement, and there could be degradation of polymer chains upon thermal treatment due to significantly increased annealing temperature and time to overcome the kinetic barrier.
Brush polymers (also called comb or graft polymers) are defined as grafted polymers with both relatively high MW and significantly dense and regularly spaced side brush chains attached to the backbone [6]. Due to the significant steric hindrance between densely grafted side brush chains, brush polymers have a highly extended backbone and exhibit a reduced degree of chain-entanglement compared to conventional polymers. Therefore, it is often favorable for brush polymers to self-assemble into well aligned and ordered nanostructures even though the MW of brush polymers is relatively high. There are three general methods to make brush polymers. In the “grafting from” approach, a macro-initiator backbone is first synthesized but there are limitations in the efficiency of its initiation and conversion of monomers. The “grafting onto” method, where the side chains and the backbone are separately synthesized and then coupled together, have difficulties in obtaining complete grafting due to increasing steric hindrance and the subsequent purification of unreacted brush side chains can be problematic [6d,6e,6g,7]. In the “grafting through” method, which is also called the “Macromonomer (MM) approach” the side chains are synthesized with a polymerizable end group which is subsequently polymerized. This approach has many advantages over those ‘graft from’ or ‘graft onto’ approaches, but still contains drawbacks like not being able to obtain high MW and/or narrow polydispersity index (PDI) [8]. Recently, Grubbs and coworkers successfully reported a novel ring-opening metathesis polymerization (ROMP) exploiting the high ring strain of norbornene monomer and the high activity of Ru-based olefin metathesis catalyst to synthesize brush polymers with ultra-high MW, narrow PDI, and well-defined, structural architectures [6f]. It is noted that, when two different brush side chains, where the difference in polymer interaction parameters is large enough to induce micro-phase segregation, are used to prepare blocky or random type of brush polymers with ultra-high MW, one, two, or three dimensionally periodic nanostructures with extremely large feature sizes can be very easily achieved, due to the significantly reduced degree of entanglements.
While BCPs have been previously used to make periodic dielectric media, the use of brush copolymers has not been extensively studied before now. There have been a couple of observations made in past publications. Bowden et al. reported in 2007 that they observed one of their block copolymers, that was combined of one grafted block and one linear block, reflecting blue light (and transmitting yellow light) and upon swelling with solvent, as is common with linear polymers, they were able to observe a red color. That system was still limited by the high degree of polymerization (≈450:2000) of the graft:linear block required to observe this optical property [6d]. Rzayev reported in 2009 that one of his brush block copolymers appeared to reflect blue light indicating interaction with visible light [6g]. In 2009 we noted that we saw one of our brush block copolymers reflect green light but no further analysis or discussion was made apart from that simple observation [6f].
It will be apparent from the foregoing description that block copolymer materials exhibiting useful physical, chemical and optical properties are useful for a range of applications including photonics, optoelectronics, and molecular templates and scaffolding. Specifically block copolymer materials are needed that are capable of efficient self-assembly to generate useful periodic structures with domain lengths in the nanometer range and exhibiting optical functionality in the visible and NIR regions of the electromagnetic spectrum.